We have identified a novel protein on the outer membrane of Arabidopsis thaliana mitochondria. This protein displays 67% sequence identity with the 64 kDa translocase of the outer envelope membrane of chloroplasts (Toc). A mitochondrial localisation for this protein was determined by (i) its presence in the proteome of highly purified Arabidopsis mitochondria, (ii) Western blot analysis with antibodies to Toc64 from pea that indicate its presence in Arabidopsis and pea mitochondria, (iii) green fluorescent protein fusion proteins that indicate an exclusive mitochondrial localisation for this protein, and (iv) expression profiles in various tissue types and during development that are more similar to translocase of the outer mitochondrial membrane components than to chloroplastic Toc components. Thus Arabidopsis mitochondria contain a protein with high sequence identity to a plastid protein import receptor. (C) 2003 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.